Cinema 4D versions come in two different licensing options, Subscription (S) and Full Release (R)
If you have an active subscription licence, you are permitted to use both the S and R versions.
If you have a perpetual licence, you are permitted to use up to the release you own.
An R21 Perpetual user can use R21 only.
An R23 Perpetual user, can use R23, S22 and R21 - they cannot use S24 or onward, as this is a subscription-only release.
A subscription customer will be able to use the latest version out, either S or R.